The reaction of uranium hexafluoride with dry steam to produce uranyl fluoride (UO.sub.2 F.sub.2) is well known and so is the reaction of the uranyl fluoride at a higher temperature with steam or hydrogen or a steam/hydrogen mixture to produce uranium oxides. It is also well known from, for example U.K. Patent Specification No. 1,320,137 and corresponding U.S. Pat. No. 3,845,193 that the two reactions may be performed sequentially in the same vessel, uranium hexafluoride and dry steam being injected together into the first region of a vessel so as to form a plume of uranyl fluoride from which uranyl fluoride is passed to the second region of the same vessel, and the uranyl fluoride reacting in the second region with a countercurrent flow of steam and/or hydrogen.